The invention relates to a hydroconversion effluent separation process wherein multiple hot and cold temperature and high and low pressure separators are used and wherein net hydrogen loss is low.
The term "hydroconversion" is used here to connote a process which consumes hydrogen and converts a hydrocarbonaceous feed, such as petroleum or a petroleum fraction, to a hydrocarbon product. Example hydroconversion processes include hydrofining, hydrotreating and hydrocracking. The term "hydroconversion" is more particularly defined hereinbelow. The present invention is particularly directed to high pressure hydroconversion processes wherein the hydroconversion reaction zone is operated at a pressure above 500 psig. Further, the present invention is particularly directed to hydroconversion processes wherein hydrogen-rich gas is recycled to the hydroconversion reaction zone.
In hydrofining, hydrotreating and hydrocracking reactions, an oil or other hydrocarbonaceous feed is upgraded by chemical reactions carried out in the presence of hydrogen gas. Hydrofining is the mildest of these three types of hydroconversion processes. The term "hydrotreating" is generally applied to more severe hydroconversion processes than hydrofining, but often is used in a broad sense to include hydrofining. Typical hydrotreating reactions include desulfurization of oil feeds and also denitrification. Heavy oil desulfurization is an important hydroconversion process and the process of the present invention is advantageously applied to such process. The term "hydrocracking" is generally used for more severe processes wherein more cracking of the oil feed occurs. However, there is not a sharp dividing line between these three types of hydroconversion processes. All three of these types of processes are well known and described in the literature, see, for example, Kirk-Othmer, Encyclopedia of Chemical Technology, Third Edition, Vol. 17, pages 201-206 and Vol. 3, page 335.
The principal chemical reactions that occur in hydroconversion processes are cracking, hydrogenation, denitrification, desulfurization, demetalation and isomerization. These reactions are typically carried out by contacting a mixture of hydrogen and the feed hydrocarbons with a catalyst contained in one or more reactors at temperatures of 400.degree. F. to 850.degree. F. and pressures of 500 to 5000 psig.
The effluent from the hydroconversion reactor contains unreacted hydrogen, converted and unconverted hydrocarbonaceous materials (mainly hydrocarbons but often also small amounts of organic sulfur and/or nitrogen compounds), and product gases. The product gases include light hydrocarbons and contaminant gases, such as H.sub.2 S and NH.sub.3, generated by the hydrogenation of sulfur and nitrogen. In broad terms, the hydroconversion reactor effluent is separated into a hydrogen-rich gas phase that is recycled to the hydroconversion reaction zone and a liquid phase that, after fractionation, may be used as a feed to further hydroconversion processes or as a feed to fluid catalytic cracking, or, especially in the case of hydrocracking, may be fractionated into liquid products, such as light naphtha, heavy naphtha, jet fuel, and diesel fuel.
The initial separation of the hydroconversion reactor effluent has normally been done by cooling the reactor effluent to temperatures that provide a gas phase with a hydrogen content suitable for recycle after removal of contaminants such as hydrogen sulfide. While such separation is an effective way to recover much of the hydrogen in the reactor effluent, it has poor energy efficiency in that a significant fraction of the heat energy in the effluent is lost. Also hydrogen losses generally are considerable.
Separation of the reactor effluent at higher temperatures has been used to improve the energy efficiency of the overall hydroconversion/separation process. Separation at higher temperatures may, however, result in a gas phase that is too hot or not sufficiently rich in hydrogen for recycle. Also, the liquid phase from a high temperature separation will contain more dissolved hydrogen, thereby increasing the net hydrogen loss of the process.
U.S. Pat. No. 4,457,834 discloses a process for separating the effluent from a hydroconversion reaction zone. According to U.S. Pat. No. 4,457,834, the recycle hydrogen is separated from effluent comprising hydrogen and hydrocarbons by a sequence of steps which includes: (a) passing the effluent to hot high pressure separation to obtain a liquid and a gas phase; (b) pressure reduction on both the liquid and gas phases to low pressure; (c) passing the lower pressure liquid phase to hot low pressure separation; (d) cooling the combined gas phases from steps (a) and (c); (e) passing the combined cooled stream to cold low pressure separation; (f) passing the liquid phase from step (e) to fractionation to obtain product hydrocarbon and passing the gas phase from step (e) to hydrogen sulfide removal to obtain purified hydrogen; (g) passing purified hydrogen to pressure swing adsorption for further hydrogen purification; (h) compressing the low pressure purified hydrogen from step (g) and recycling the compressed hydrogen to the hydroconversion reactor. In the process of U.S. Pat. No. 4,457,834, although much of the hydrogen dissolved in the liquid from the hot high pressure separator is recovered for recycle, the recycle hydrogen requires considerable energy input for compression, since all the recycled hydrogen in the '834 process derives from a low pressure separator.
The use of membranes to separate hydrogen from other gases has been disclosed in various references, such as "Separation Systems For Oil Refining and Production", W. A. Bollinger et al, Chem. Engr. Progress, October 1982, page 27; "High Hydrogen Purity Is Key in New Refining Era", E. A. Maciula, Oil and Gas Journal, May 1980, page 63; "Is Permeation Competitive?", R. C. Schendel et al, August 1983, page 58; "Du Pont Membrane System Recovers Hydrogen", Chem. and Engr. News, Apr. 14, 1986, page 24; U.S. Pat. No. 4,398,926; "Enhanced Hydrogen Recovery From Low Purity Gas Streams", August, 1983; U.S. Pat. No. 4,362,613; "Hydrocracking Processes Having an Enhanced Efficiency of Hydrogen Utilization", December, 1982; and "Optimizing Hydrocracker Hydrogen", W. A. Bollinger et al, Chem. Engr. Progress, May 1984, page 51.
The "Optimizing Hydrocracker Hydrogen" article discloses a hydrogen separation system for use in conjunction with a hydrocracking reaction. Effluent from the hydrocracker is cooled and passed to a high (1800 psig) pressure separator where hydrogen is separated for recycle and liquid hydrocarbons are withdrawn for passage to a low pressure separator. Since the hydrogen is recycled, the temperature in the high pressure separator is likely to be in the area of 120.degree. F. (cold high pressure separation to obtain reasonable purity of hydrogen for recycle). The recycle hydrogen is brought back to the hydrocracker via a recycle compressor. A portion of the hydrogen from the high pressure separator is withdrawn from the recycle loop as a high pressure purge stream. The high pressure purge stream is passed to a membrane separation unit "Prism separator" for recovery of high purity hydrogen from the purge stream.
A system such as described in the "Optimizing Hydrocracker Hydrogen" article cannot successfully be used in residua hydrotreating. Amongst other factors, a water wash would be needed ahead of the cold high pressure separator and this would not operate satisfactorily because of emulsion formation and foaming.